An optical scan device having an optical deflector reflecting a light beam from a light beam source so as to deflect the light beam and having a surface to be scanned on which information is written such that the light beam deflected by the deflector scans the surface is disclosed. Optical detectors are arranged at least in two locations, a start side of writing and an end side of writing, which locations are outside an effective writing area. A measuring part measures a scan time required by the light beam deflected by the optical deflector to scan a range between the optical detectors. A correcting part corrects each dot position of image data in the effective writing area to an arbitrary position based on a variation amount of the measured scan time.